Fuel injectors provide a way to introduce fuel into the combustion chambers of an engine. One type of fuel injector is known as the common rail fuel injector. A typical common rail fuel injector includes a nozzle assembly having a cylindrical bore with a nozzle outlet at one end, and a nozzle supply passageway in communication with a high pressure fuel rail at an opposite end. A needle check valve is reciprocatingly disposed within the cylindrical bore and spring-biased toward a closed position at which the nozzle outlet is blocked. To inject fuel, the needle check valve is moved to open the nozzle outlet, thereby allowing high pressure fuel to travel from the high pressure rail through the nozzle supply passageway and spray into the associated combustion chamber.
One way to move the needle check valve between the open and closed positions includes draining and filling a control chamber associated with a base of the needle check valve. In particular, the control chamber may be filled with pressurized fuel to retain the needle check valve in a closed position, and selectively drained of the pressurized fuel to bias the needle check valve toward the open position.
A piezo device is often hydraulically coupled to the control chamber to affect draining and filling of the control chamber. Specifically, the piezo device is typically mechanically connected to a first piston, which is separated from a second piston by a space filled with fuel known as a hydraulic coupling. The hydraulic coupling is used to accommodate manufacturing tolerances, heat expansion of the injector components, and/or amplification of force or movement of the piezo device. As the piezo device is charged and expands to move the first piston, the fuel pressure of the hydraulic coupling increases, resulting in movement of the second piston. The second piston then presses against and opens a control valve, thereby draining the control chamber. As long as the hydraulic coupling remains pressurized to the correct pressure, expansion and contraction of the piezo device result in accurate fuel injection events. However, if fuel leaks from the hydraulic coupling and is not replenished, movement of the piezo device can result in undesired or no movement of the control valve.
One example of replenishing the hydraulic coupling is described in U.S. Pat. No. 6,840,466 (the '466 patent) issued to Igashira et al. on Jan. 11, 2005. The '466 patent describes a common rail fuel injector having a check valve installed on a lower end of the first piston. The check valve works to compensate for a loss of fuel due to leakage, by connecting a sump with a displacement amplifying chamber (e.g., the hydraulic coupling described above). The check valve consists of a flat valve closing a passage in the first piston between the sump and the displacement amplifying chamber, and a conical spring urging the flat valve upwards to block the passage. The flat valve is made of a thin disc, which has a pinhole formed in the center thereof. The pinhole serves to allow the leakage of fuel from within the displacement amplifying chamber to the sump in the event of a failure during injection, thereby stopping the injection. The pinhole also serves as a vacuum in the displacement amplifying chamber for removing bubbles from the chamber.
Although the flat check valve included with the fuel injector of the '466 patent may sufficiently replenish fuel leaked from the displacement amplifying chamber, it may have limited application. In particular, because the flat check valve includes a hole through which fuel may leak during injection events, it may be difficult to build significant pressure within the displacement amplifying chamber. In fact, as described in the '466 patent, the hole may even act as a vacuum, directly acting against the buildup of pressure within the chamber. This reduced level of pressure may limit control valve movement and/or force amplification, and the resulting injection pressure available from the injector. In addition, even if significant pressure buildup was possible within the injector of the '466 patent, the flat nature of the check valve may provide too little support against the pressure, possibly resulting in deformation of the check valve and/or failure of the injector.
An alternative embodiment of the injector of the '466 patent is disclosed in SAE TECHNICAL PAPER SERIES 2006-01-0174, entitled “180MPa Piezo Common Rail System.” As illustrated in FIG. 6 of this paper, the above-described fuel injector is fitted with a more robust full ball check valve, instead of the flat check valve described in the '466 patent.
Although the full ball check valve may be more robust and, thus, may withstand greater pressures, it may still be problematic. In particular, the full ball check valve may require a greater volume to accommodate the increased size of the full ball check valve. This increased volume may add to the volume of the displacement amplifying chamber, which must be pressurized by the downward displacing movement of the first piston. If the displacing movement of the piezo device is kept the same, the greater volume will result in a lower pressure within the chamber. If the displacing movement of the piezo device is increased, component cost and size of the injector must also be increased.
The control system of the present disclosure solves one or more of the problems set forth above.